<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<script>
    window.onload = function(){
        /*720代表设计师给的设计稿的宽度，你的设计稿是多少，就写多少;100代表换算比例，这里写100是
          为了以后好算,比如，你测量的一个宽度是100px,就可以写为1rem,以及1px=0.01rem等等*/
        getRem(375,100);
    };
    window.onresize = function(){
        getRem(375,100);
    };
    function getRem(pwidth,prem){
        var html = document.getElementsByTagName("html")[0];
        var oWidth = document.body.clientWidth || document.documentElement.clientWidth;
        html.style.fontSize = oWidth/pwidth*prem + "px";
    }
</script>
<body>
<div style="width:100%;height:15rem;background:red;">
<div style="position:fixed;width:30px;height:30px;background:green;left:50%;top:90%;"></div>
</div>
</body>
</html>